Per The Unofficial Apple Weblog, online backup service CrashPlan co-founder Matthew Dornquist had the following to offer about the new Java vulnerability and what it could mean for the Mac.

In a recent study of a random sample of 200K recent users; Dornquist’s numbers showed that the overwhelming majority of CrashPlan’s Mac users are on Java 1.6 (92%) and a small minority on the older 1.5 version. The percentage on the 1.7 version targeted by the malware? Approximately zero.

Research shop FireEye identified a Java zero-day exploit this weekend that is already targeting fully patched versions of the Java JRE version 1.7 running on Windows machines. The exploit attempts to install a dropper executable (Dropper.MsPMs) on the machines it attacks. In theory, a separate dropper could be crafted to attack Mac or Linux systems, although none has yet been observed in the wild.

That’s a reason for Mac users to rest a little more easily, but it’s not the big one. As noted by CNET, the vulnerable edition of the JRE — 1.7 — isn’t installed by default in a stock configuration of OS X. The Java that Apple delivers on Snow Leopard, Lion and Mountain Lion is JRE 1.6 (and on Lion and Mountain Lion, it’s only installed on demand when needed to run Java applications); in order to be on 1.7 and be theoretically susceptible, you’d have to install the Oracle beta build manually.

If you did install the Oracle build and you’re concerned about the new exploit, you can disable the Java plugin in each of your browsers individually, or uninstall 1.7 entirely. While it bears repeating that there is no evidence of a Mac payload for this exploit at this time, if you don’t have a specific reason to run the new version then it’s probably safest to stick with JRE 1.6 instead (or turn off Java completely if you don’t need it). In response to past exploits including Flashback, Apple’s Java web plugin is now set to auto-disable when it isn’t used for some time, further reducing the attack surface for Mac users.

So, yeah, try to avoid manually updating to Java 1.7 on your Mac until this is sorted out and we’ll have additional details as they become available.